A known high-frequency heating unit in a related art adjusts power supplied to a magnetron according to the width of the output pulse from an inverter control circuit. As the output voltage of signal superposition means becomes higher, the output pulse width of the inverter control circuit widens and the power supplied to the magnetron increases. This configuration makes it possible to change the output voltage of the signal superposition means for continuously changing the heating output of the magnetron.
Since a heater also serves as a cathode of the magnetron, a transformer for supplying power to the magnetron also supplies power to the heater and thus the power supplied to the heater also changes in response to change in the power supplied to the magnetron. Thus, if an attempt is made to maintain the heater temperature in an appropriate range, the heating output can be changed only in a slight range and it is impossible to continuously change the heating output; this is a problem.
As a high-frequency heating unit to solve this problem, a control system disclosed in the patent document 1 is available. FIG. 30 is a diagram to describe a high-frequency heating unit for executing the control system. In FIG. 30, the heating control system includes a magnetron 701; a transformer 703 for supplying power to a heater 715 of the magnetron 701 at the same time as supplying high-voltage power to a high-voltage rectifying circuit 702 for supplying secondary winding power to the magnetron 701; an inverter circuit 705 for rectifying an AC power supply 704, converting it into predetermined-frequency AC, and supplying the AC to the transformer 703; power detection means 706 for detecting input power to or output power from the inverter circuit 705; an output setting section 707 for outputting an output setting signal corresponding to any desired heating output setting; a power regulation section 708 for making a comparison between the output of the power detection means 706 and the output setting signal and controlling the DC level of a power regulation signal so as to provide any desired heating output; oscillation detection means 719 for outputting an oscillation detection signal which makes a LOW to HIGH transition if the output of the power detection means 706 becomes equal to or larger than an output level of reference voltage generation means 718; a comparison voltage generation circuit 716 for generating a voltage corresponding to the output setting signal, a waveform shaping signal comparing the output setting signal by a level conversion circuit 720; a waveform shaping circuit 721 for shaping output of a rectifying circuit 710 for rectifying the AC power supply voltage 704 based on the waveform shaping signal and the oscillation detection signal; a comparison circuit 711 for comparing an output signal of the waveform shaping circuit 721 with output of the comparison voltage generation circuit 716 and outputting a comparison reference voltage when the former is smaller than the latter or executing inverting amplification when the former is larger than the latter; signal superposition means 712 for superposing a fluctuation signal of output of the comparison circuit 711 on the power regulation signal and outputting a pulse width control signal; an oscillation circuit 713, and an inverter control circuit 714 for executing pulse width modulation of output of the oscillation circuit 713 by the pulse width control signal and driving the inverter circuit 5 according to the modulation output.
The high-frequency heating unit regulates the power supplied to the magnetron 701 based on the output pulse width of the inverter control circuit 714. As the output voltage of the signal superposition means 712 becomes higher, the output pulse width of the inverter control circuit 714 widens and the power supplied to the magnetron 701 increases. In the unit, the output voltage of the signal superposition means 712 is changed continuously, whereby it is made possible to continuously change the heating output of the magnetron 701.
According to the configuration, shaping is performed in response to the output setting by the waveform shaping circuit 721 for inputting the rectification voltage of the AC power supply 704 and outputting to the comparison circuit 711. Inverting amplification of the output of the waveform shaping circuit 721 is performed by the comparison circuit 711 having the comparison voltage generation circuit 716 for generating a reference signal at the level corresponding to the heating output setting signal as a reference voltage and the inverting amplification signal and the output of the power regulation section 708 are superposed on each other, whereby as for the pulse width control signal output by the signal superposition means 712, the level in the vicinity of the maximum amplitude of the AC power supply 704 becomes lower and the level in the magnetron non-oscillation portion becomes higher at the low output time as compared with the time when the heating output setting is high output and thus the oscillation period per power supply cycle of the magnetron is prolonged. Accordingly, the power supplied to the heater increases. Further, at the high output time, the input current waveform of the inverter becomes a waveform which is upward convex in the envelope peak vicinity and is close to the shaped waveform of a sine wave, and harmonic current is suppressed.
Thus, the pulse width control signal is controlled so that the heater current is much entered at the low output time and power supply current harmonic lessens at the high output time by the waveform shaping circuit 721, whereby the power supply current harmonic can be kept low, change in the heater current can be made small, and a highly reliable high-frequency heating unit can be realized.
However, in the control, it turned out that waveform shaping cannot follow up variations or types of characteristics of magnetrons, ebm (anode-cathode voltage) fluctuation caused by the temperature of an anode of a magnetron and the load in a microwave oven, or power supply voltage fluctuation because waveform shaping based on “prospective control system” is executed so that the input current waveform becomes close to a sine wave by performing pulse width modulation using a modulation waveform provided by processing and shaping a commercial power supply waveform for an ON/OFF drive pulse of a switching transistor.
The variations and types of characteristics of magnetrons motivating the invention will be briefly discussed. Since VAK (anode cathode voltage)-Ib characteristic of a magnetron is nonlinear load as shown in FIG. 31, the ON width is modulated in response to the phase of commercial power supply and the input current waveform is brought close to a sine wave for improving the power factor.
The nonlinear characteristic of the magnetron varies depending on the type of magnetron and also fluctuates due to the magnetron temperature and the heated substance (load) in the microwave oven.
FIG. 31 is anode cathode application voltage-anode current characteristic drawings of magnetrons; (a) is a drawing to show the difference depending on the magnetron type; (b) is a drawing to show the difference depending on good and bad of matching of power supply of magnetrons; and (c) is a drawing to show the difference depending on the magnetron temperature. In the drawings, (a) to (c), the vertical axis indicates anode-cathode voltage and the horizontal axis indicates anode current.
Then, referring to (a), A, B, and C are characteristic drawing of three types of magnetrons. For the magnetron A, only a slight current of IA1 or less flows until VAK becomes VAK1 (=ebm). However, if VAK exceeds VAK1, current IA rapidly starts to increase. In this region, IA largely changes with a slight difference of VAK. Next, for the magnetron B, VAK2 (=ebm) is lower than VAK1 and for the magnetron C, VAK3 (=ebm) is further lower than VAK2. Since the nonlinear characteristic of the magnetron thus varies depending on the magnetron type A, B, C, for a modulation waveform matched with a magnetron with low ebm, the input current waveform becomes distorted when a magnetron with high ebm is used. The units in the related arts cannot deal with the problems. Then, producing a high-frequency dielectric heating circuit not affected by the magnetron type is a problem.
Likewise, referring to (b), the characteristic drawing of the three types of magnetrons shows good and bad of heating chamber impedance matching viewed from each magnetron. If the impedance matching is good, VAK1 (=ebm) is the maximum and becomes smaller as worse. Thus, the nonlinear characteristic of the magnetron also varies largely depending on whether the impedance matching is good or bad and therefore producing a high-frequency dielectric heating circuit not affected by the magnetron type is a problem.
Likewise, referring to (c), the characteristic drawing of the three types of magnetrons shows high and low of the temperatures of the magnetrons. If the temperature is low, VAK1 (=ebm) is the maximum and as the temperature becomes gradually higher, ebm becomes lower. Therefore, if the magnetron temperature is matched with a low temperature, when the magnetron temperature becomes high, the input voltage waveform becomes distorted.
Thus, the nonlinear characteristic of the magnetron also varies largely depending on the magnetron temperature difference and therefore producing a high-frequency dielectric heating circuit not affected by the magnetron type is a problem.
Patent document 2 discloses a control system to deal with the problems described above. FIG. 32 is a block diagram to describe a high-frequency heating unit for executing the control system.
In FIG. 32, AC voltage of an AC power supply 220 is rectified by a diode bridge type rectifying circuit 231 made up of four diodes 232 and is converted into a DC voltage through a smoothing circuit 230 made up of an inductor 234 and a capacitor 235. Then, the DC voltage is converted into a high-frequency AC by an inverter circuit made up of a resonance circuit 236 made up of a capacitor 237 and a primary winding 238 of a transformer 241 and a switching transistor 239, and a high-frequency high voltage is induced in a secondary winding 243 of the transformer 241 through the transformer 241.
The high-frequency high voltage induced in the secondary winding 243 is applied between an anode 252 and a cathode 251 of a magnetron 250 through a voltage multiplying rectifier 244 made up of a capacitor 245, a diode 246, a capacitor 247, and a diode 248. The transformer 241 also contains a tertiary winding 242 for heating the heater (cathode) 251 of the magnetron 250. The described circuitry is an inverter circuit 210.
Next, a control circuit 270 for controlling the switching transistor 239 of the inverter will be discussed. First, current detection means 271 of a CT, etc., detects an input current to the inverter circuit, a rectifying circuit 272 rectifies a current signal from the current detection means 271, a smoothing circuit 273 smoothes the signal, and a comparison circuit 274 makes a comparison between the signal and a signal from an output setting section 275 for outputting an output setting signal corresponding to heating output setting. Since the comparison circuit 274 makes a comparison to control the magnitude of power, in place of the above-described input signal, an anode current signal of the magnetron 250, a collector current signal of the switching transistor 239, or the like may be an input signal.
On the other hand, the AC power supply 220 is rectified through a diode 261 and a shaping circuit 262 shapes the waveform. Then, an inversion and waveform processing circuit 263 inverts a signal from the shaping circuit 262 and performs waveform processing. A gain variable amplifier circuit 291 (described later) varies the output signal from the shaping circuit 262 and outputs a reference waveform signal. A waveform error detection circuit 292 outputs a difference between the input current waveform signal from the rectifying circuit 272 and the reference waveform signal from the gain variable amplifier circuit 291 as a waveform error signal. A mix and filter circuit 281 (which will be hereinafter referred to as “mix circuit”) mixes and filters the waveform error signal from the waveform error detection circuit 292 and a current error signal from the comparison circuit 274 and outputs an ON voltage signal. A comparison is made between the ON voltage signal and a sawtooth wave from a sawtooth wave generation circuit 283 in the comparator 282 and pulse width modulation is performed for controlling turning on/off of the switching transistor 239 of the inverter circuit.
FIG. 33 shows an example of the mix circuit 281. The mix circuit 281 has three input terminals; an auxiliary modulation signal is applied to a terminal 811, the waveform error signal is applied to a terminal 812, and the current error signal is applied to a terminal 813. The signals are mixed in an internal circuit as shown in the figure. Numeral 810 denotes a high-frequency cut filter having a function of removing a high frequency component of the current error signal whose high frequency component is not required. If a high frequency component exists, when the current error signal is mixed with the waveform error signal, the fluctuation of the waveform error signal is not finely output.
As described above, the variable amplifier circuit 291 automatically creates the waveform reference following the magnitude of the input current, the waveform error detection circuit 292 makes a comparison between the waveform reference and the input current waveform provided by the current detection means 271 and provides waveform error information, and the provided waveform error information is mixed with output of input current control for converting into an on/off drive signal of the switching transistor 239 of the inverter circuit for use.
Thus, a control loop operates so that the input current waveform matches the waveform reference following the magnitude of the input current. Therefore, if there are variations in the types and the characteristics of magnetrons or if there is ebm (anode-cathode voltage) fluctuation caused by the temperature of the anode of the magnetron and the load in the microwave oven or further if there is power supply voltage fluctuation, input current waveform shaping not affected by them is made possible.    Patent document 1: JP-A-7-136375    Patent document 2: JP-A-2004-30981